In a quick review of Jerry Mac's book it looked as if some of the "C Stock class" were wearing shoulder belts. I'm guessing it was based on the speed or ETs. Jerry Mac. should be able to answer this question if he gets on-line, since his 68 Camaro was raced back and still today. D Stock Eliminator if I remember correctly and still races today.

Below is a clip from a search from the 1968 NHRA rules, its not very clear unless you dive into which class...

10:11 SEAT BELTSAll cars not required by Class Requirements to use SFI 16.1 driverrestraint systems must be equipped with an accepted quick-release typedriver seat belt. Belts must be securely fastened to the frame,cross member, or reinforced mounting so that all fittings are in adirect line with the direction of pull. Seat belts may not be wrappedaround lower frame rails. Steel castings of the type recommended byFAA or U-bolt-type mounts are permitted. If used for installation, flatsteel plates must be a minimum of 1/4-inch thickness and haverounded edges to prevent cutting seat belts. Under nocircumstances can belts be installed with bolts through webbing. Inall cars with fiberglass floors, a cross member (minimum 2-inch x 2-inch x .083-inch wall thickness square tubing) must be installedbetween frame rails for proper driverís seat-belt installation.

So, I'm voting no for D Stock and lower classed that were run back then, but I could be wrong again..